Improvement to eager linear regression benchmark

Before:
entry {
  name: "EagerLinearRegressionBenchmark.eager_train_cpu"
  iters: 2000
  wall_time: 2.45178794861
  extras {
    key: "examples_per_sec"
    value {
      double_value: 52206.7987456
    }
  }
}

After:
entry {
  name: "EagerLinearRegressionBenchmark.eager_train_cpu"
  iters: 2000
  wall_time: 1.9873790741
  extras {
    key: "examples_per_sec"
    value {
      double_value: 64406.4344182
    }
  }
}
PiperOrigin-RevId: 188068838